1. Field of the Invention
This invention relates to stepping motors, and more particularly to a winding for operating a three-phase stepping motor from a two-phase drive, such as to produce a step angle other than that which would normally be produced by the motor.
2. Background Art
As is well known, a stepping motor is a device which translates electrical pulses into mechanical movements by causing an output shaft to move a specific incremental distance or "step" for each pulse applied to the motor. As opposed to a conventional motor which has a free-running shaft, the stepping motor shaft moves in fixed, repeatable, known increments, resulting in the ability to accurately position. These motors are controlled by drive circuitry which provides the necessary number, sequence, and rate of pulses to achieve the desired extent of movement.
Stepping motors including synchronous inductor motors of the general type considered here have been described in detail elsewhere, as, for example, in U.S. Pat. No. 3,117,268. There, a rotary stepping motor has a permanent magnet moving member, or rotor, and a stationary member, or stator, comprising a plurality of poles with windings thereon for magnetizing the poles. Both the rotor and the stator have their magnetically coacting peripheries toothed and, when a unidirectional current is selectively applied to energize specific poles, the rotor can be made to rotate a predetermined increment (a "step"). Switching means in the drive circuitry are provided to continuously step the rotor in either direction in response to input control commands or to maintain the rotor at a hold, or stop, position which is the position to which it was last moved.
Very commonly, such a motor has eight stator poles and is a two-phase motor with four of the stator poles connected in one phase and the other four stator poles connected in the other phase, thus resulting in two identical phases with four magnetic poles each, typically referred to as a "four-pole" motor. This type of motor requires a two-phase drive for operation. Such a motor typically produces a mechanical stepping angle of 1.8.degree.. This is determined as follows:
A stepping motor of the type under consideration here usually has 50 teeth around the periphery of the rotor and a stator tooth pitch of 48, 50, or 52 and the rotor is made to rotate in a chosen direction by the following four-step drive sequence, with "positive" and "negative" referring to the polarity of the current in the given phase:
______________________________________ Phase 1 Phase 2 ______________________________________ Step 1 +1 +1 Step 2 +1 -1 Step 3 -1 -1 Step 4 -1 +1 ______________________________________
This sequence rotates the rotor an angular distance of one rotor tooth pitch. Thus, the step angle is equal to 360.degree./(50 rotor teeth)/(four steps)=1.8.degree. mechanical. Since the electrical field producing flux in the stator rotates through 360.degree. for the movement of the rotor one tooth pitch, each step is equivalent to a 90.degree. change in the electrical field.
It is well known that as the number of stator poles of a stepping motor is increased, there can be a corresponding increase in the torque produced by the motor and, also, smoother operation can be achieved. For the embodiment of the present invention described below, a twelve pole stator was selected. Considerations of commonality dictated that a 50-tooth rotor and a 48-, 50-, 52-, or variable-tooth-pitch stator be used; and, likewise, that the motor produce a step angle of 1.8.degree. mechanical when operated with a two-phase drive, to accommodate existing two-phase drives and applications. Simply operating a twelve-pole stepping motor as a three-phase motor with a three-phase drive, would produce a step angle of 1.2.degree. mechanical.